The object of the present invention is a method and device used in pulp web drying, as defined in the preambles to the independent claims presented below.
This means that the object of the present invention concerns especially a method and device used in drying a pulp web in a pulp dryer, comprising at least two, or typically several drying levels arranged on top of each other, which drying levels consist of two rows of blow boxes with their nozzle surfaces facing each other, between which a drying gap is formed. From the blow boxes drying air or the like is blown onto the pulp web passing through the drying gap. The passage of the pulp web from one drying level to another is guided by means of turning rolls. In this type of dryer, the wet pulp web is first carried to the topmost drying level of the pulp dryer, and from there on as a winding web through the other drying levels of the pulp dryer, after which the dried pulp web is carried out of the pulp dryer.
Air drying has become established as a common method for drying the pulp web after the press section. In the air dryer the pulp web or web passes in a winding manner through levels that are arranged one on top of the other, from the upper part of the dryer to its lower part. On each level, the web passes horizontally from one end of the dryer to the other, supported on an air-cushion formed by the air blown from the nozzles, without contact with the nozzles, and is turned at the end by the turning rolls to the next drying level. The length of one drying level may be 20-50 m, typically 35-40 m, and the number of levels may be 10-30, typically 18-26. Thus the drying distance on the largest machines is well over 1 km.
The pulp web is transferred from the press section to the dryer by an open draw, that is, without being supported. The pulp dryer, which is structurally higher than the other parts of the wet end, usually has to be installed on a level lower than the machine level of the wet end, on the so-called basement level, from where the dryer rises to machine level and from there several meters, for example 7 m, above machine level. The last press of the press section from which the wet pulp web is lifted to the upper part of the dryer, is usually located only about 2 m above machine level, which means that the wet web has to be lifted by an open draw some 5 m in order to bring the web to the first drying level in the upper part of the pulp dryer.
Pulp production lines are under continuous pressure to increase production and improve efficiency. This leads to an increase in the speed of the machines and in grammages of the pulp web, which in turn results in increasingly heavy pulp webs having to be lifted from the press section to the topmost drying level. The dry weight of the pulp web on pulp dryers falls generally within the range of 550-1200 g/m.sup.2. A heavy, wet pulp web may easily break when lifted, and the heavier the pulp is and/or the higher the web has to be lifted, the greater the problems. Difficulties due to heavy grammages may already appear when the dry weight exceeds 900 g/m.sup.2. On the other hand, lifting the web to a height exceeding 5 meters may in itself be difficult, irrespective of the grammage.
If the web is wet and heavy, tail threading in particular will be difficult, that is, it is difficult to thread the end of the pulp web undamaged into the dryer. Successful threading is, however, important from the point of view of the efficiency of the production line, to ensure rapid start-up. As pulp web grammages increase, the air blown should be able to carry and transfer increasingly heavy webs at increasing speeds in the dryer itself, which might be difficult, especially on the uppermost drying level through which the pulp web passes when it is at its wettest, that is, at its heaviest.
Another general trend in paper and pulp production is that an increasing number of new, exotic chemical pulps are being tried out and introduced into production. Some of these new pulps are considerably more difficult to dewater on the wire section than. conventional pulps, which means that the pulp will then come from the press section to the air dryer in a wetter state, that is heavier, than normal, thus causing problems. These short-fibred pulps are often also weak in other ways too, and threading them through the dryer may be extremely difficult. In such cases web breaks usually take place immediately on the topmost drying level of the dryer.
Runnability problems in the air dryer due to an excessively heavy web arise even if the actual dry weight of the web is within the correct range, if the web comes to the dryer wetter than expected, that is heavier than expected, due to the operation of the wet end of the pulp machine not being quite in order. In such a case, problems may arise even with familiar long-fibred pulps. Problems may, therefore, arise due to the malfunctioning of the wet end, but only appear as actual breaking problems on the topmost drying level of the pulp dryer. Roughly speaking, it can be said that the situation becomes more difficult if the dry matter content of the web remains below 46%. Apart from the web then being clearly heavier than it would be if its dry matter content was, for example 50%, the wetter the pulp is, the poorer the wet strength of the web.
Variations in dry matter content may also be due to the different dewatering properties of the pulps used. Different pulp types have, among other things, different dewatering properties on the wire section, which is why the dry matter content and thus also the wet strength before the dryer varies by type of pulp.
Problems due to an excessively wet or heavy web generally arise on the topmost level of the dryer, where the air-cushion blown by the blow nozzles under the web is no longer able to carry the heavier web and the web begins to drag along the nozzles, which sooner or later results in a web break. Then knots of fibre become detached from the web--from the layer of the web that has dried first--and these knots are then rolled into so-called cigars, or dust will form, both of which will sooner or later cause a web break and other problems which will result in a fall in production. When the web breaks, the "cigars" and other pulp stock, especially the pulp stock at the wet end of the dryer, easily get caught between the blow boxes, thus making it more difficult to clean the dryer. The risk of these web breaks is greatest on the topmost levels of the pulp dryer.
It is in fact the aim of the present invention to present an improved method and device for pulp web drying, in which particularly the above-mentioned problems have been minimised.
The aim of the invention is thus particularly to achieve a more sustainable lift of a wet or heavy pulp web than before--also in connection with tail threading--from the press to the pulp dryer.
A further aim of the invention is to improve the passage of a particularly wet or heavy pulp web in the first part of the pulp dryer.
For achieving the above aims, the method and device relating to the invention are characterised by what is defined in the characterising parts of the independent claims.
In a pulp dryer, the device relating to the invention comprises a support wire or the like which supports the passage of the pulp web to the pulp dryer, at least to the start of its topmost drying level or the immediate vicinity of the start, but which may also be arranged to support the passage of the pulp web through the first part of the pulp dryer, typically only through the first, or topmost, drying level. If necessary, it is obviously possible to arrange additional support wires on the pulp dryer, which also support the passage of the pulp web through the second and third drying levels. The support wire acting as the leading wire of the pulp web may be very coarse, typically much coarser than the wet end wire used in pulp machines. The support wire can be made of conventional wire materials, either in metal or with a plastic base. The choice of material is affected, for example, by the temperature of the drying process used at any given time.
A typical pulp dryer comprises at least two, or usually a considerable number, such as 10-30, drying levels arranged on top of each other and running from one end of the pulp dryer to the other, looking in the direction of travel of the web. The wet pulp web is carried to the topmost drying level, from where it will pass on in a winding manner through the dryer, one drying level after another, down to the lower part of the dryer. The pulp web is guided by means of the turning rolls at each end of the dryer from one drying level to another, that is, to the next, lower drying level.
The drying levels consist of two rows of blow boxes or the like with their nozzle surfaces facing each other. The space between the rows of blow boxes forms the pulp web drying gap, in which hot drying air or other similar hot gas is blown from the blow boxes onto the pulp web, in order to dry the pulp web as it passes through the drying gap. The length of the drying levels is usually 20-40 meters, typically 35-40 meters, which means that each drying level is fitted with some 130-160 blow boxes, half of which are top nozzles and half bottom nozzles.
Various types of blow boxes known as such can be used in pulp dryers for drying the pulp web and for its non-contacting support through the pulp dryer. Below the pulp web, nozzles are normally used, through which nozzles at least some of the air is discharged as jets parallel to the bearing surface of the blow boxes. The purpose of these air jets parallel to the plane of the pulp web is mainly to stabilise the non-contacting passage of the pulp web through the drying level. Above the pulp web, on the other hand, so-called impingement nozzles are normally used, from which nozzles air is discharged mainly in blows directed perpendicularly towards the pulp web. From the point of view of drying, the perpendicular air jets are the most important. In the dryer, the type of blow boxes disclosed in the American patent U.S. Pat. No. 5,471,766 can, for example, advantageously be used.
In the solution relating to this invention in which the support wire is arranged to support the passage of the pulp web also through the first drying level, the blow boxes of the above type that are located below the support wire, can be replaced by impingement boxes, that is, by means of boxes from which hot air is discharged mainly perpendicularly towards the pulp web. When the pulp web is supported and carried by the support wire, air blows parallel to the plane, which stabilise the web, are not needed, and neither would these blows facilitate drying because they would remain mainly below the support wire and would not, therefore, come into contact with the pulp web to be dried. The entire amount of air can be directed through the support wire, perpendicularly to the pulp web in order to maximise the drying effect of the air.
When the support wire supports the pulp web, the blow from above the support wire can be intensified in comparison to what is conventional without endangering the passage of the pulp web. This will compensate for the fact that the support wire has the effect of weakening the flow coming from below the support wire, that is, the effect of impairing the drying of the bottom surface of the pulp web. On the other hand, intensifying the blowing of the hot air flow from above enhances the drying of the top surface of the pulp web considerably, which facilitates the passage of the web on the next drying level, where this intensively dried top side of the web is in turn the supporting underside. The intensively dried underside is considerably less liable to adhere to the bearing surface of the blow boxes than a wet surface from which fibres detach easily, forming dust and the above-mentioned "cigars".
Sometimes at least some of the blow boxes below the support wire can be replaced by means that support the support wire mechanically, such as sliding surfaces or rollers.
Furthermore, a support wire passing through the first drying level prevents pulp stock from penetrating between the blow boxes below the support wire in the event of a web brake. The support wire conveys the stock formed during the break to the other end of the pulp dryer, out of reach of the blow boxes, from where it may fall down freely.
The advantages achieved by means of the invention include the following:
The support wire supports the pulp web in the free space between the press and the dryer, supporting the tail threading carried out by a normal tail threading means. In tail threading, the support wire guides the pulp web, which broadens out in width from a narrow band having the width of the leader threading cord to the full width of the web, safely through the "crisis area", that is, the rise and, if necessary, the first drying level. Having passed the crisis area, the web has dried and is stronger and can proceed on its own. PA1 The support wire makes possible the passage of webs which are heavier/wetter than usual through "crisis areas", that is, the rise to the upper part of the dryer and, if necessary, through the first drying level. PA1 The support wire then also makes possible the use of higher and steeper rises from the press to the upper part of the pulp dryer. PA1 A support wire extending to the first drying level of the pulp dryer makes possible the use of even more efficient blow boxes in the dryer. PA1 The use of the support wire also makes it possible to achieve more efficient drying above the web, on the first drying level, which facilitates the passage of the pulp web on the drying levels to follow and reduces the formation of "cigars". PA1 The support wire prevents pulp stock from being caught between the blow boxes and thus reduces cleaning time and expenses. PA1 The support wire makes production possible even if the dewatering of the wet end of the pulp machine does not correspond to conventional dewatering due to a technical structural solution or, for example, variation in pulp quality.